Time interval measurements have been made by counting the number of pulses of a timing signal which occurred between the start and stop events. Also, various interpolating schemes have been devised to determine the time interval between the start and stop event with an accuracy greater than .+-.1 count of the timing pulses. See, for example, U.S. Pat. No. 3,133,189 issued to A. S. Bagley, et al on May 12, 1964 and entitled "Electronic Interpolating Counter For The Time Interval And Frequency Measurement".
A single vernier method has been used wherein the time between a trigger pulse and the next available clock pulse is measured in much the same way as the fraction of a graduation as indicated in a pair of vernier calipers. A trigger pulse starts an oscillator of period T.sub.0 [1+(1/N)] which beats against the clock period T.sub.0 as shown in FIG. 1. The time interval between the input trigger pulse and the "next" clock pulse can be determined by counting the number of pulses between the trigger and the point of coincidence. In the example of FIG. 1, this number is 4 (the unknown interval is proportional to this number). Coincidence between the vernier and the clock is detected, terminating the vernier count N.sub.1. The time interval between the trigger signal and the next clock is given by (T.sub.0 N.sub.1)/N.
These previous techniques had to overcome the severe problem of maintaining the frequency accuracy of the start and stop oscillators when first turned on, since the measurement is extremely sensitive to the frequency stability of the vernier signal (the main clock or time base is assumed to be stable). The interpolation factor of N requires stability of the order of 1/N.sup.2 under all conditions. Also, sub-nanosecond coincidence resolution required sub-nanosecond detection techniques. Circuits used in these techniques were required to be able to identify the "next" clock pulse without .+-.1 count ambiguity. The determination of the coincidence of the start and stop pulses with the time base was frequently ambiguous when the start or stop pulse occurred very near a clock pulse. Also, occurrence of the start pulse may not always precede the stop pulse in some measurements, but they may be random with respect to each other. Vernier methods generally offer no solutions to these problems.